1. Field of the Invention
The present invention relates to a fuel cell including a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates, as well as a polymer electrolyte membrane that forms an electrolyte layer of a polymer electrolyte fuel cell.
2. Description of the Related Art
Fuel cells generally have a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates. In the fuel cells, reactions expressed by Equations (1) and (2) given below proceed respectively on an anode (hydrogen electrode) and a cathode (oxygen electrode).
Anode (Hydrogen Electrode)
H2xe2x86x922H++2exe2x80x83xe2x80x83(1)
Cathode (Oxygen Electrode)
(xc2xd)O2+2H++2exe2x86x92H2Oxe2x80x83xe2x80x83(2)
The hydrogen ion produced on the hydrogen electrode is hydrated to form hydroxonium ion (xH2O)H+ and shifts to the oxygen electrode through the electrolyte layer.
A diversity of fuel cells with various electrolyte layers have been proposed: phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and alkali fuel cells. Much attention has been drawn to polymer electrolyte fuel cells using a hydrogen ion-conductive polymer membrane as the electrolyte layer, because of the potential for high output density and size reduction. Various techniques have been studied to improve the properties of such fuel cells.
The fuel cells with any electrolyte layers generate electricity, based on the above principle. The theoretical electromotive force, that is, the theoretical potential difference between the hydrogen electrode and the oxygen electrode, is approximately 1.23 V. In the actual conditions, the output voltage is lowered to approximately 0.95 to 1 V, due to a variety of losses. One of the main factors to decrease the output voltage is the internal resistance, that is, the resistance caused by the low mobility of hydrogen ions in the electrolyte layer.
A diversity of techniques have been proposed to reduce the internal resistance in the polymer electrolyte fuel cells; for example, the techniques disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 6-231781, No. 8-171920, and No. 7-135004. The techniques disclosed in the former two applications vary the water content of the polymer electrolyte membrane formed as the electrolyte layer in such a manner that the water content on the side of the hydrogen electrode is higher than the water content on the side of the oxygen electrode. As mentioned previously, the hydrogen ions are hydrated or combined with water molecules to form the hydroxonium ions, while shifting through the electrolyte layer. With a progress in reaction, water molecules become insufficient on the side of the hydrogen electrode that supplies the hydrogen ions, while becoming excess on the side of the oxygen electrode. The proposed techniques give a difference in water content between the two electrodes, so as to cancel the shortage of water molecules and facilitate the smooth shift of the hydrogen ions.
The technique disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 7-135004 increases the concentration of the ion exchange group contained in the electrolyte layer. The hydrogen ions and the hydroxonium ions shift through the electrolyte layer with the aide of the ion exchange groups. The increase in concentration of the ion exchange group accordingly decreases the internal resistance. JAPANESE PATENT LAID-OPEN GAZETTE No. 7-135004 also discloses the technique that makes the concentration of the ion exchange group on the side of the hydrogen electrode higher than that on the side of the oxygen electrode. The higher concentration of the ion exchange group generally improves the water absorption capacity. The higher concentration of the ion exchange group on the side of the hydrogen electrode than that on the side of the oxygen electrode accordingly increases the water content on the side of the hydrogen electrode. This ensures the similar effects to those attained by the techniques disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 6-231781 and No. 8-171920 described above.
These proposed techniques aim to reduce the internal resistance to improve operation efficiency of the fuel cells, but not to enhance the electromotive force of the fuel cells. The reduction of the internal resistance slightly enhances the output voltage. But the improved level still remains at about 1 V against the theoretical, maximum electromotive force of approximately 1.23 V.
In the event that fuel cells are used as the power source of various apparatuses, the fuel cells are expected to output the required voltage according to each apparatus. The low electromotive force per unit cell causes an increase in the number of unit cells connected to output the required voltage. The greater number of unit cells undesirably makes the whole power source system bulky and increases the manufacturing cost. From this point of view, the enhancement of the electromotive force of the fuel cells is very important. The proposed techniques have been mainly directed to the reduction of the internal resistance to improve the operation efficiency, but there has been no fully discussion on the enhancement of the electromotive force.
These problems arise not only in polymer electrolyte fuel cells but in other types of fuel cells.
An object of the present invention is thus to provide a technique that enhances electromotive force of a fuel cell.
Another object of the invention is to provide an electrolyte membrane that is applied for a polymer electrolyte fuel cell having an enhanced electromotive force.
At least part of the above and the other related objects is attained by a fuel cell including a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates. The electrolyte layer has a first contact area, where the electrolyte layer is in contact with the oxygen electrode, and a second contact area, where the electrolyte layer is in contact with the hydrogen electrode. The hydrogen ion concentration in the first contact area is higher than the hydrogen ion concentration in the second contact area. It is preferable that the difference of the ion concentration is a predetermined value corresponding to a target electromotive force on an occasion of power generation.
The fuel cell of this arrangement has the enhanced electromotive force, due to the difference in hydrogen ion concentration between the side of the hydrogen electrode and the side of the oxygen electrode. The fuel cell of the present invention is preferably used as the unit cell of a power source system. This desirably decreases the required number of unit cells to output the required voltage, thereby reducing the size and the manufacturing cost of the whole power source system.
The following describes the relationship between the variation in hydrogen ion concentration and the electromotive force. The electromotive force of the fuel cell represents the potential difference between the hydrogen electrode and the oxygen electrode. The reactions expressed by Equations (1) and (2) given above proceed on the respective electrodes. The reactions occurring at the respective electrodes are in an equilibrium state in the process of power generation. The potentials at the respective electrodes in the equilibrium state are generally expressed by the Nernst equation. According to the Nernst equation, the equilibrium electrode potential EH at the hydrogen electrode and the equilibrium electrode potential EO at the oxygen electrode are expressed respectively by Equations (3) and (4) given below.                                                                         E                H                            =                              xe2x80x83                            ⁢                                                E                  H0                                +                                                      (                                          RT                      /                      F                                        )                                    xc3x97                                      ln                    ⁡                                          (                      aH                      )                                                                                                                                              =                              xe2x80x83                            ⁢                                                E                  H0                                -                                                      (                                          RT                      /                      F                                        )                                    xc3x97                  pH                                                                                        (        3        )                                                                                    E                O                            =                              xe2x80x83                            ⁢                                                E                  O0                                +                                                      (                                          RT                      /                      F                                        )                                    xc3x97                                      ln                    ⁡                                          (                      aH                      )                                                                                                                                              =                              xe2x80x83                            ⁢                                                E                  O0                                -                                                      (                                          RT                      /                      F                                        )                                    xc3x97                  pH                                                                                        (        4        )            
where R denotes the gas constant, T denotes the absolute temperature or Kelvin temperature, F denotes the Faraday constant, aH denotes the activity of hydrogen ion, EH0 represents the potential (0 V) at the hydrogen electrode under the condition of aH=1, EO0 represents the potential (1.23 V) at the oxygen electrode under the condition of aH=1, and pH (hydrogen ion exponent) is equal to xe2x88x92ln(aH).
The activity of hydrogen ion aH is not strictly identical with the hydrogen ion concentration in some cases. In the specification hereof, however, it is assumed that the activity of hydrogen ion aH is identical with the hydrogen ion concentration.
FIG. 1 is a graph showing variations in equilibrium electrode potentials at the hydrogen electrode and the oxygen electrode plotted against the pH value. Namely this graph represents Equations (3) and (4) given above according to Nernst equation. The solid line represents a variation in equilibrium electrode potential at the oxygen electrode, whereas the broken line represents a variation in equilibrium electrode potential at the hydrogen electrode. As shown in the graph of FIG. 1, each electrode potential decreases with a decrease in hydrogen ion concentration. The potential difference between the two electrodes is fixed to 1.23V at each pH value, as clearly understood from Equations (3) and (4). For example, when the electrolyte layer has pH=1, the electrode potential at the oxygen electrode is equal to Vo1 and the electrode potential at the hydrogen electrode is equal to Vh1. The electromotive force of the fuel cell at this point corresponds to the potential difference between the hydrogen electrode and the oxygen electrode, that is, a voltage V1, and is theoretically equal to 1.23 V. As far as the electrolyte layer has a homogeneous composition on both the sides of the hydrogen electrode and the oxygen electrode, the theoretical value of the electromotive force is 1.23 V, irrespective of the pH of the electrolyte layer.
The fuel cell of the present invention has the difference in hydrogen ion concentration, that is, pH (hydrogen ion exponent), between the part of the electrolyte layer close to the oxygen electrode and the part of the electrolyte layer close to the hydrogen electrode. The hydrogen ion concentration on the side of the hydrogen electrode is lower than that on the side of the oxygen electrode. One example of this state is shown in FIG. 1. In the fuel cell of the present invention, for example, when the pH on the side of the oxygen electrode is equal to pHo, the pH on the side of the hydrogen electrode is equal to pHh, which is higher than pHo. The electrode potential at the oxygen electrode is accordingly equal to Vo2, and the electrode potential at the hydrogen electrode is equal to Vh2. The electromotive force of the fuel cell corresponds to the potential difference between the two electrodes, that is, a voltage V2. As clearly understood from the graph of FIG. 1 and Equations (3) and (4), the voltage V2 is greater than 1.23 V. The electromotive force varies according to the pH difference between the side of the oxygen electrode and the side of the hydrogen electrode.
In the conventional fuel cells, a common electrolyte layer is used for the hydrogen electrode and the oxygen electrode. As described previously, the proposed technique simply gives a difference in water content between the side of the hydrogen electrode and the side of the oxygen electrode, in order to improve the hydrogen ion conductivity. The inventors of the present invention have given a preference to the enhanced electromotive force of the fuel cell, based on the principle of power generation of the fuel cell. The inventors have then given the attention to the relationship expressed by Nernst equation, that is, the relationship between the pH value of the electrolyte layer and the potentials at the respective electrodes, and completed the invention, based on the finding that the variation in pH value between the two electrodes enhances the electromotive force of the fuel cell. The Nernst equation itself is well known in the art. The technical significance of the present invention is that the Nernst equation is reexamined with a view to enhancing the electromotive force of the fuel cell and that the difference in pH between the part of the electrolyte layer close to the hydrogen electrode and the part of the electrolyte layer close to the oxygen electrode leads to the enhancement of the electromotive force.
As clearly understood from the graph of FIG. 1 and Equations (3) and (4), the technique of the present invention sets the difference between the pH value on the side of the hydrogen electrode and the pH value on the side of the oxygen electrode according to the target electromotive force. The graph of FIG. 1 shows the theoretical values. In the actual state, the difference between the pH values on the sides of the two electrodes is set while taking into account the voltage drop due to a variety of losses. The technique of the present invention is based on the finding that the equilibrium state of the reactions proceeding on the hydrogen electrode and the oxygen electrode varies with a variation in pH of the electrolyte layer. The hydrogen ion concentration of the electrolyte layer is accordingly required to have a variation in contact areas, where the electrolyte layer is in contact with the respective electrodes, and more strictly in regions affecting the equilibrium state of the reactions proceeding on the respective electrodes. The hydrogen ion concentration of the electrolyte layer may be increased gradually from the side of the hydrogen electrode to the side of the oxygen electrode. As far as the above conditions are fulfilled, any value may be set to the hydrogen ion concentration in the electrolyte layer.
The technique of the present invention is applicable to a variety of fuel cells, such as phosphoric acid fuel cells and molten carbonate fuel cells. It is, however, especially preferable that the electrolyte layer is a hydrogen ion exchange membrane, which is mainly composed of a solid polymer. Namely the technique of the present invention is preferably applied to polymer electrolyte fuel cells.
The polymer electrolyte fuel cell has an electrolyte layer composed of a polymer membrane. No discussion has been made to adopt the technique of varying the composition of the electrolyte layer between the two electrodes. The inventors have overthrown the conventional idea and clarified the importance of the varying composition of the electrolyte layer between the two electrode. This is the significance of the present invention. It is relatively easy to vary the composition of the electrolyte layer between the two electrodes, since the electrolyte layer is composed of a polymer membrane. The polymer electrolyte membrane advantageously maintains the difference between the pH value on the side of the hydrogen electrode and the pH value on the side of the oxygen electrode over a relatively long time.
The difference in pH value between the side of the hydrogen electrode and the side of the oxygen electrode may be attained by a variety of arrangements.
In accordance with one preferable embodiment, the electrolyte layer has a varying concentration of an ion exchange group for the hydrogen ions in such a manner that the concentration of the ion exchange group in the first contact area, where the electrolyte layer is in contact with the oxygen electrode, is higher than that in the second contact area, where the electrolyte layer is in contact with the hydrogen electrode.
The hydrogen ion generally shifts from the hydrogen electrode to the oxygen electrode by the function of the ion exchange group. As is known in the art, the hydrogen ion concentration depends upon the concentration of the ion exchange group, and a greater number of hydrogen ions are present in the area including a greater number of ion exchange groups. In the fuel cell of the above arrangement, the first contact area, where the electrolyte layer is in contact with the oxygen electrode, has a higher concentration of the ion exchange group. This makes the hydrogen ion concentration on the side of the oxygen electrode higher than that on the side of the hydrogen electrode. This enhances the electromotive force. The concentration of the ion exchange group is set arbitrarily as far as the relationship between the two electrodes satisfies the above condition. The concentration of the ion exchange group on the side of the hydrogen electrode may be decreased, or alternatively the concentration on the side of the oxygen electrode may be increased. The difference in concentration of the ion exchange group between the respective electrodes is set according to the target electromotive force. The varying concentration of the ion exchange group may be attained by different quantities of the ion exchange group to be contained in the sides of the respective electrodes in the course of preparing the membrane of the electrolyte layer. The varying concentration may alternatively be attained by joining a pair of polymer membranes having different concentrations of the ion exchange group with each other.
In a concrete example of this arrangement, the electrolyte layer is a hydrogen ion exchange membrane, which is mainly composed of a sulfonic acid group-containing perfluorocarbon polymer, and the ion exchange group is sulfonic acid group. The ion exchange group is, of course, not restricted to the sulfonic acid group, but a diversity of other groups, for example, phosphoric acid group, may be applied for the ion exchange group.
This arrangement is generally applied for the polymer electrolyte fuel cell and is known so far as the preferable materials for the polymer electrolyte fuel cell having the excellent driving efficiency and durability. The combination of this arrangement with the technique of the present invention gives the fuel cell having the enhanced electromotive force, in addition to the variety of conventionally improved properties. In the fuel cell having this arrangement, varying the concentration of the sulfonic acid group in a region of not greater than 1 xcexcm relative to the electrolyte layer having the thickness of several tens micrometer ensures the sufficient effects. Setting the concentration ratio of the sulfonic acid group of the side of the hydrogen electrode to the side of the hydrogen electrode equal to approximately 1 to 10 enhances the electromotive force by 20 to 50 mV. The concentration ratio may be set arbitrarily according to the target electromotive force.
In accordance with another preferable embodiment, the technique of the present invention is applied for the polymer electrolyte fuel cell. The electrolyte layer contains a non-proton cation, in such a manner that concentration of the non-proton cation in the vicinity of the second contact area, where the electrolyte layer is in contact with the hydrogen electrode, is higher than that in the vicinity of the first contact area, where the electrolyte layer is in contact with the oxygen electrode.
In the fuel cell of this arrangement, the Coulomb force between the non-proton cation and the hydrogen ion functions as the repulsive force on the side of the hydrogen electrode. The repulsive force works to keep the hydrogen ions away from the hydrogen electrode. This causes the hydrogen ion concentration to be lowered on the side of the hydrogen electrode and thereby enhances the electromotive force of the fuel cell. A diversity of non-proton cations may be contained in the electrolyte layer; for example, sodium ion (Na+), potassium ion (K+), calcium ion (Ca2+), and silver ion (Ag+). The techniques generally adopted in the process of forming a catalyst layer on the electrode may be applied to make the cations contained in the electrolyte layer. One applicable method impregnates the electrolyte layer with a solution containing a salt of the cation and removing only the non-required anion in an environment of high temperatures.
The technique of the present invention may be attained by an arrangement outside the electrolyte layer, in place of the above arrangement in the electrolyte layer.
The present invention is accordingly directed to a fuel cell including a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates. The fuel cell further has enhancement element that increases a hydrogen ion concentration with the electrode layer during power generation in such a manner that the hydrogen ion concentration in a first contact area, where the electrolyte layer is in contact with the oxygen electrode, is higher than the hydrogen ion concentration in a second contact area, where the electrolyte layer is in contact with the hydrogen electrode, by at least a predetermined value corresponding to a target electromotive force.
In the fuel cell of this arrangement, the enhancement element causes a difference in hydrogen ion concentration between the first contact area close to the oxygen electrode and the second contact area close to the hydrogen electrode. The difference in hydrogen ion concentration desirably enhances the electromotive force. A diversity of techniques may be applied for the enhancement element. For example, the electrical field may be applied to induce or repel the hydrogen ion. In another example, surface treatment of the electrolyte layer with chemicals may be performed to cause the difference in hydrogen ion concentration. This arrangement advantageously allows the use of the electrolyte layer having the conventional structure and enhances the electromotive force by a relatively simple process.
In accordance with one concrete embodiment of the fuel cell having the characteristics outside the electrolyte layer, the electrolyte layer is a hydrogen ion exchange membrane, which is mainly composed of a solid polymer. At least one of the hydrogen electrode and the oxygen electrode has a specific structure that is partly in contact with the electrolyte layer. The enhancement element causes a difference in hydrogen ion conductivity between a contact region in the electrolyte layer, where the at least one electrode having the specific structure is in contact with the electrolyte layer, and a non-contact region.
The hydrogen ion conductivity is an index representing the ease of the movement of hydrogen ion. The higher conductivity represents the lower resistance to the movement of hydrogen ion. In the fuel cell of the above arrangement, there is a difference in hydrogen ion conductivity between the contact region of the electrolyte layer that is in contact with the electrode and the non-contact region. This leads to a difference in distribution of the hydrogen ion between the contact region and the non-contact region. For example, the hydrogen ion conductivity is raised in the contact region, where the electrolyte layer is in contact with the oxygen electrode, and lowered in the non-contact region. This makes a dense distribution of hydrogen ion shifted to the oxygen electrode in the contact region. The electrode reaction actually proceeds in the contact area. The variation in distribution of hydrogen ion accordingly has the similar effects as the increased hydrogen ion concentration in the contact area close to the oxygen electrode. On the contrary, the hydrogen ion conductivity may be raised in the non-contact region and lowered in the contact region, where the electrolyte layer is in contact with the hydrogen electrode. In this case, the non-contact region has a dense distribution of hydrogen ion produced at the hydrogen electrode. This has the similar effects as the decreased hydrogen ion concentration in the contact area close to the hydrogen electrode. The fuel cell of the above arrangement accordingly causes a difference in hydrogen ion concentration between the contact region and the non-contact region. This arrangement thus enhances the electromotive force, like the arrangement of varying the hydrogen ion concentration between the electrodes.
A diversity of techniques may be applied to cause a difference in hydrogen ion conductivity between the contact region and the non-contact region.
In accordance with one preferable embodiment, the oxygen electrode has the specific structure that is partly in contact with the electrolyte layer, and the enhancement element is a water repellent layer provided on surface of the non-contact region in the electrolyte layer.
The hydrogen ion is generally hydrated or combined with water molecules to form hydroxonium ion, when shifting through the electrolyte layer. The presence of water molecules thus significantly affects the hydrogen ion conductivity. The water repellent layer provided in the non-contact region close to the oxygen electrode keeps the water molecules away from the non-contact region and thereby reduces the number of water molecules existing in the non-contact region. The water molecules kept away from the non-contact region naturally concentrate in the contact region. The greater number of water molecules in the vicinity of the contact region enhances the hydrogen ion conductivity in the contact region, whereas the non-contact region has the lowered hydrogen ion conductivity.
A water repellent layer provided in the contact region close to the hydrogen electrode, where the hydrogen electrode is in contact with the electrolyte layer, has the same effects as those of the above arrangement. Similar effects are also expected by providing a hydrophilic layer in the contact region close to the oxygen electrode or in the non-contact region close to the hydrogen electrode. Among these substantially equivalent arrangements, the water repellent layer is readily formed in the non-contact area of the electrolyte layer close to the oxygen electrode. In this arrangement, the water repellent layer is obtained by simply applying a fluoro compound on the surface of the electrolyte layer or coating the surface with a fluoro compound. Another advantage is that the water repellent layer does not interfere with the electrode reaction in the contact region.
In the fuel cell of the present invention, it is preferable that the first contact area, where the electrolyte layer is in contact with the oxygen electrode, is narrower than the second contact area, where the electrolyte layer is in contact with the hydrogen electrode.
The reaction on each electrode actually proceeds in the contact area, where the electrolyte layer is in contact with the electrode. The narrow contact area suppresses the reaction. Namely the narrow contact area interferes with the smooth shift of hydrogen ion between the electrolyte layer and the electrode. The wider contact area, on the other hand, facilitates the reaction and accelerates the smooth shift of hydrogen ion between the electrolyte layer and the electrode. In the fuel cell of the above application, the first contact area, where the electrolyte layer is in contact with the oxygen electrode, is narrower than the second contact area, where the electrolyte layer is in contact with the hydrogen electrode. This arrangement interferes with the smooth shift of hydrogen ion at the oxygen electrode, so as to raise the hydrogen ion concentration in the second contact area, while accelerating the smooth shift of hydrogen ion at the hydrogen electrode, so as to lower the hydrogen ion concentration in the first contact area. This causes a difference in hydrogen ion concentration between the first contact area and the second contact area and thus enhances the electromotive force, based on the functions discussed above. The effect of the electromotive force enhancement by this arrangement is not so significant, and it is accordingly preferable to combine this arrangement with any of the applications discussed above.
In the case of a polymer electrolyte fuel cell, the technique of the present invention may be attained by a polymer electrolyte membrane included in the fuel cell.
The present invention is accordingly directed to a polymer electrolyte membrane that forms an electrolyte layer of a fuel cell, wherein the fuel cell includes a hydrogen electrode and an oxygen electrode disposed across the electrolyte layer, which hydrogen ion permeates. The electrolyte layer contains an ion exchange group for the hydrogen ions in such a manner that concentration of the ion exchange group in a second contact area, where the electrolyte layer is in contact with the hydrogen electrode, is lower than that in a first contact area, where the electrolyte layer is in contact with the oxygen electrode.
The present invention is further directed to a polymer electrolyte membrane that forms an electrolyte layer of a fuel cell, wherein the fuel cell includes a hydrogen electrode and an oxygen electrode disposed across the electrolyte layer, which hydrogen ion permeates. The electrolyte layer contains a non-proton cation in the vicinity of a contact area, where the electrolyte layer is in contact with the hydrogen electrode.
Application of the polymer electrolyte membrane of this arrangement for the fuel cell causes a difference in hydrogen ion concentration between a contact area of the polymer electrolyte membrane close to the hydrogen electrode and a contact area close to the oxygen electrode, based on the functions discussed above. This desirably enhances the electromotive force. The variety of arrangements utilizing the additional elements discussed above with regard to the fuel cell are also applicable to the polymer electrolyte membrane of the present invention.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.